This project is directed at determining the role of action potential activity in the development of proper synaptic connections in the visual system. The experimental animal used is the newborn kitten. Action potentials are silenced by repeated injection of tetrodotoxin (TTX) into one eye. This blockade is reversible; when it is allowed to wear off, connections between retinal ganglion cells and lateral geniculate nucleus (LGN) relay cells are assessed. Single unit electrophysiological recording techniques are the major means used to characterize receptive field properties of LGN neurons. We have already determined that prolonged TTX blockade causes many indiscriminate connections to be made between the silenced retinal axons and LGN relay cells. For example, totally abnormal ON/OFF type relay cells are in the majority in the deprived LGN layers. This represents a gross disruption of normal development. Also, many binocularly innervated units are found at the borders on normal and deprived LGN layers. This is the first demonstration that nerve cell activity, per se, is a factor in determination of neural interconnections in the visual system. The work proposed here has two main aims: 1. to further characterize the results of action potential absence on visual system development; 2. to attempt to understand the mechanism by which action potential activity influences proper synaptic development.